The present invention relates to a Ti alloy poppet valve and a method of manufacturing the same.
To decrease inertial mass to improve engine performance, intake and exhaust valves in an internal combustion engine are made of Ti alloy instead of heat resistant steel. But Ti is likely to be combined with another element such as oxygen and wear resistance is not sufficient.
On the surface of Ti alloy poppet valve, nitriding and oxidizing as disclosed in Japanese Patent No. 3,022,015, carburizing as disclosed in U.S. Pat. No. 5,466,305 or Ni plating is applied to increase wear resistance.
A valve to which nitriding or oxidizing is applied provides sufficient wear resistance, but has too high hardness, so that it is likely to attack other members. It is necessary to change material of the valve-operating part which is engaged with the valve, so that cost increases.
During oxidizing, a workpiece is placed at high temperature, 750 to 800xc2x0 C. in atmosphere to which air or oxygen is supplied, so that diffusion of oxygen is too fast, thereby forming hard fragile oxide layer such as TiO2 and Ti2O3, which is likely to be separated.
It is difficult to attain sufficient wear resistance by carburizing on the surface of the valve. In a valve to which Ni plating is applied, heat resistance is not sufficient and it is not suitable to employ it as exhaust valve.
In view of the disadvantages as above, it is an object of the invention to provide a Ti alloy poppet valve in which wear resistance is significantly increased without forming titanium oxide.
It is another object of the invention to provide a method of manufacturing a Ti poppet alloy valve in which wear resistance is significantly increased.
According to one aspect of the invention, there is provided a Ti alloy poppet valve which consists of a valve stem and a valve head, said valve having a surface layer which comprises an oxygen diffusion layer of an interstitial solid solution of O in Ti.
According to another aspect of the invention, there is provided a method of manufacturing a Ti alloy poppet valve, said method comprising the steps of:
introducing O2 into a furnace to keep oxygen density less than stoichiometrical amount for forming titanium oxides in the furnace; and
heating the valve for 1 to 4 hours at temperature of 700 to 840xc2x0 C. to introduce oxygen atoms into titanium of the valve to form a Tixe2x80x94O interstitial solid solution, thereby increasing wear resistance of the valve.
If the temperature is less than 700xc2x0 C., oxygen is not sufficiently diffused into the Ti alloy valve, and required hardness is not obtained. If the temperature is more than 840xc2x0 C., the poppet valve is deformed and is not actually employed as product. The range of 750 to 800xc2x0 C. is preferable.
If the time is less than 1 hour, required hardness is not obtained, and if more than four hours, treating time is too long and productivity of the valve is decreased. The range of 2 to 3 hours is preferable.
The oxygen density to a surface area of the valve may be preferably 1.10xc3x9710xe2x88x927 g/cm2 to 1.47xc3x9710xe2x88x926 g/cm2. If it is less than 1.10xc3x9710xe2x88x927 g/cm2, hardness is not sufficient, and if it is more than 1.47xc3x9710xe2x88x926 g/cm2, oxygen is combined with Ti to form titanium oxide.
By the poppet valve manufactured by the present invention, wear resistance and durability are increased.